Who knows what it means to describe a woman's bun?

This comes from a joke that the nursing home for women is very small.

There is a lady whose breasts are so small that few people propose to her. One day, a handsome guy proposed to her and asked: Your breasts must be bigger than steamed bread, right? The lady nodded her head. On the wedding night, as soon as he entered the bridal chamber, the man rushed out of the door angrily and cursed: "Wang Zi steamed bread is also called steamed bread."